In your project you have dotenv listed as a dependency or a devDependency which opts-out of Next.js' automatic loading to prevent creating a conflict with any existing dotenv behavior in your project.
This is also disabled if a package.json isn't able to found in your project somehow.
Update to the latest version of Next.js (>= v9.4.1) where this support is enabled regardless of dotenv being installed.
Remove dotenv from your devDependencies or dependencies and allow Next.js to load your dotenv files for you.